Using Heat to Cool Buildings

Novel materials could make practical air conditioners and refrigerators that use little or no electricity.

It could soon be more practical to cool buildings using solar water heaters and waste heat from generators. That’s because of new porous materials developed by researchers from the Pacific Northwest National Laboratory. These materials can improve a process called adsorption chilling, which can be used for refrigeration and air conditioning.

Hot pack: A display at a conference shows a new material (light green) packed into a metal foam. The material is being used to improve a technology that uses heat energy to drive a cooling process.

Adsorption chillers are too big and expensive for many applications, such as use in homes. Peter McGrail, who heads the research effort, predicts that the materials could allow adsorption chillers to be 75 percent smaller and half as expensive. This would make them competitive with conventional, compressor-driven chillers.

All refrigerators and air conditioners cool by evaporating a refrigerant, a process that absorbs heat. They differ in how that refrigerant is condensed so that it can be reused for cooling. Unlike the technology inside most air conditioners, which employs electrically driven compressors to mechanically compress the vaporized refrigerant, adsorption chillers use heat to condense the refrigerant. Adsorption chillers are typically far less efficient than chillers that use electrical compressors, and are bulky and expensive. But they have the advantage of being cheap to operate, since they require very little electricity. “If you have waste heat, you can run it for free,” McGrail says.

So far these chillers have been limited to applications where there is a lot of waste heat—such as industrial facilities and power plants—or where electricity isn’t always available. Cutting their size and cost could make them attractive in more applications, including in homes, where they could be run using hot water from solar heaters, McGrail says.

The key is improving the solid adsorbent material. In an adsorption chiller, evaporated refrigerant is adsorbed—it adheres to a surface of a solid, such as silica gel. The silica gel can hold a large amount of water in a small space—it essentially acts as a sponge for the water vapor. When the gel it heated, it releases the water molecules into a chamber. As the concentration of water vapor in the chamber increases, the pressure rises until the water condenses.

McGrail is replacing silica gel with an engineered material made by creating nanoscopic structures that self-assemble into complex three-dimensional shapes. The material is more porous than silica gel, giving it a larger surface area for water molecules to cling to. As a result, it can trap three to four times more water, by weight, than silica gel, which helps reduce the size of the chiller.

The material also binds less strongly to water molecules. That reduces the amount of heat needed to free the water molecules—making the process more efficient—and speeds up the process of adsorbing and desorbing water by 50 to 100 times, which helps make the chiller smaller. The materials also work with refrigerants other than water, which expands the temperature range at which cooling is possible.

Since current adsorption chillers can be two or three times larger than chillers that use electric compressors, “cutting the size of adsorption chillers by 75 percent could make them competitive,” says Yunho Hwang, a professor at the Center for Environmental Energy Engineering at the University of Maryland. The chillers could be particularly useful for cooling with hot water from solar water heaters, since adsorption chillers can use the relatively low-temperature such heaters produce, he says.

One challenge for such applications could be synchronizing demand for cooling with the production of heat—in some cases, it may be necessary to include a costly heat-storage system to make it possible to keep the chiller running after the sun goes down.

The PNNL researchers have been awarded $2.54 million from the Advanced Research Projects Agency for Energy to demonstrate the material in a cooling system. Under the grant, they have three years to optimize the material’s performance and incorporate it into a small demonstration chiller.

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